Just a whiff of penguin poop can send krill into a frenzy

Imagine the world as if you are a krill in the Southern Ocean, where any moment a torpedo-shaped penguin could appear, devouring thousands of your fellow krill at lightning speed. Now, new research suggests that even the faintest hint of penguin droppings in the water is enough to prompt krill into escape behavior.

“Here we show for the first time that a small amount of penguin guano causes a sudden change in the feeding and swimming behaviors of Antarctic krill,” said Nicole Hellessey, a postdoctoral researcher at the Bigelow Laboratory for Ocean Sciences in Maine.

Antarctic krill play a vital role in the Southern Ocean. They serve as a major carbon sink and provide food for a large array of species, from penguins to whales.

By some estimates, approximately 700 trillion adult Antarctic krill still swim these frigid waters, though their populations have drifted further south over time, likely due to climate change, diminishing sea ice, and acidifying ocean conditions.

Linking penguin poop to krill behavior

Zooplankton are highly sensitive to chemical cues – from signals guiding them to food or potential mates to indicators of pollution. Hellessey and her colleagues sought to determine whether krill also react to the scent of predators. 

Their study zeroed in on Adélie penguins, the southernmost-breeding penguin species, which depend on Antarctic krill for 99.6% of their diet. An adult Adélie can consume as much as 1.6 kilograms of krill each day, and the global Adélie population collectively devours around 1.5 million tons per year.

Researchers aboard the vessels Laurence M Gould and Nathaniel B Palmer in the Bransfield Strait in late 2022 netted live Antarctic krill, which they then placed in aquaria at Palmer Station.

Meanwhile, bird researchers had collected 78 grams of Adélie guano from a colony on Torgersen Island off the Antarctic Peninsula near Palmer Station. “Smells like rotten shellfish. Not pleasant to handle,” said Hellessey, describing the guano.

Abrupt behavior shifts in krill

To test the krill’s reaction to the guano, the scientists ran trials using six to eight krill per five-minute session in a flume tank held at 1.5°C, with lights dimmed to simulate conditions at about 40 meters below the surface – the most productive layer in the Southern Ocean. 

Water flowed through the flume at either 3 centimeters or 5.9 centimeters per second, and it contained either algae alone, penguin guano alone, or both combined. Each setup was tested four times.

The researchers used two automated cameras to record the movements of four krill during each trial, capturing precise data on each animal’s 3D position as well as speed and swimming direction.

The data revealed that in control settings, krill generally practiced “rheotaxis,” swimming straight upstream against the current.

But in the presence of Adélie guano, their behavior shifted abruptly: they began to vary their swimming speed more, reaching between 1.2 and 1.5 times their usual pace, and they performed three times as many turns – over an angle that was on average 1.4 times greater than before.

Krill make sacrifices to avoid penguins

A second experiment showed that guano in the water caused krill to reduce their algae consumption by 64%, plunging from 12.7 micrograms of carbon per hour per krill to 4.6 micrograms.

This indicates that the frequent changes in swimming direction came at a cost, making them less efficient at feeding.

The researchers interpreted these dramatic differences in speed and direction as part of an avoidance response.

“Such behavior to escape from nearby penguins would greatly increase the krill’s odds of survival. And these odds would increase exponentially in a swarm, if their neighbors could detect the same cues and communicate the danger to each other,” Hellessey said.

Investigating the chemical trigger

Though the team uncovered the behavioral shift, it is still unclear which specific compounds from penguin guano set off these alarms for krill. 

“We hypothesize that Antarctic krill are avoiding the odor of ground-up krill and fish in the penguins’ guano. We thus expect krill to show similar swimming behaviors and suppressed feeding around seals, whales, and other types of krill predators in Antarctica,” Hellessey noted.

Krill signals in a warming ocean

Given that these signals and their resulting behaviors occur in the often vast Southern Ocean, open-water conditions could dilute the cues and alter how krill respond. Moreover, warming oceans and acidification may alter how effectively these cues travel or how quickly the krill can respond. 

“We don’t yet know how the ability of krill to sense these chemical cues and their escape behavior towards them might vary when diluted in open waters, or under global warming or ocean acidification conditions,” said Hellessey.

“Any changes to krill’s behavior could have major impacts on the future Southern Ocean, as Antarctic krill are a keystone species in this ecosystem.”

Predicting broader ecological shifts

Ultimately, this study adds an important layer to our knowledge of predator-prey interactions in polar waters. As climate shifts reshape the Southern Ocean, understanding how krill perceive and react to predator signals may help predict broader ecological shifts. 

If krill alter their movements and feeding drastically, it could influence entire marine food webs and carbon cycling processes – a reminder that even simple chemical cues can produce outsized effects on oceanic ecosystems.

The study is published in the journal Frontiers in Marine Science.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–